NOx generally refers to a mixture of nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O). The lifespan of NOx components in the atmosphere typically ranges from one to seven days for NO and NO2, and to 170 years for N2O, NO has no color, odor, or taste, and is relatively non-toxic. In the presence of air, NO is rapidly oxidized to NO2. NO2 is a reddish brown pollutant with high reactivity. NO2 is also a strong oxidizing agent that can form the corrosive nitric acid, as well other toxic organic nitrates. Further, NO2 is a well-known pulmonary irritant affecting the human respiratory system, which may exacerbate asthma and other respiratory conditions. Still further, NO2 absorbs light, is a main component of smog, and is sometimes seen as the yellow-brown haze hanging over cities or as the yellow plume in the stack downstream of a gas turbine. In such instances, NO2 may be found in concentrations above 10-12 ppm.
Numerous technologies exist to reduce NOx components in gaseous streams, particularly exhaust streams from the combustion of fuel. Many of those solutions primarily focus on the reduction of the NOx components down to N2. Moreover, NO2 tends to be more abundant in exhaust streams at start up, lower temperatures, and part loads. The reduction of NO2 to NO(NO2→NO+½O2), however, is poorly understood and controlled. In one study, Bamwenda et al., React. Kinet. Catal. Lett. 63:1 pp. 53-59 (1998) examined the reduction of NO2 by C3H6 in the presence of O2 over alumina-supported Au, Rh and Pt. The authors, however, only achieved a reduction efficiency of up to 50% at 400° C. before gradually falling off with temperature. Further, a drawback of injecting a substantial amount of an organic compound into an exhaust stream is the formation of hydrocarbons and other byproducts that need to be treated in order to comply with environmental regulations, as well as any excess of the organic compound itself in the stream. Improved solutions for reducing NO2 to NO are thus needed to reduce NO2 to NO with higher efficiency without generating significant pollutants.